boundary layer control of an axial compressor … · performed to predict the effect of inserting...

1 Copyright © 2015 by ASME

Proceedings of the ASME 2015 International Mechanical Engineering Congress & Exposition

IMECE2015

November 13-19, 2015, Houston, Texas, USA

IMECE2015-52310

BOUNDARY LAYER CONTROL OF AN AXIAL COMPRESSOR CASCADE USING

NONCONVENTIONAL VORTEX GENERATORS

Ahmed M.Diaa Assiut University

Assiut, Egypt [email protected]

Mohammed F. El- Dosoky Assiut University

Assiut, Egypt mffe1 @ aun.edu.eg

Mahmoud A. Ahmed Assiut University


Omar E. Abdel-Hafez Assiut University


ABSTRACT Boundary layer control plays a decisive role in controlling

the performance of axial compressor. Vortex generators are well

known as passive control devices of the boundary layer. In the

current study, two nonconventional types of vortex generators

are used and their effects are investigated. The used vortex

generators are doublet, and wishbone. Three dimensional

turbulent compressible flow equations through an axial

compressor cascade are numerically simulated. Comparisons

between cascade with and without vortex generators are

performed to predict the effect of inserting vortex generator in

the overall performance of the axial compressor. Results

indicate that using vortex generators leads to eliminate or delay

the separation on the blade suction surface, as well as the

endwall. Furthermore, the effects of the vortex generators and

their geometrical parameters on the aerodynamic performance

of the cascade are documented. In conclusion, while the

investigated vortex generators cause a slight increase in the total

pressure loss, a significant reduction in the skin friction

coefficient at the bottom endwall is found. This reduction is

estimated to be about 46% using doublet and 32% using

wishbone.

INTRODUCTION The importance of axial compressors due to its

relevance to gas turbine applications has motivated many

researchers toward enhancing its overall performance.

Controlling the secondary flow phenomena associated with the

flow in compressor cascades will significantly improve the

aerodynamic performance of compressors. This is due to the

fact that secondary flows are extracting energy from the

fluid and increasing the flow instability. Endwall boundary

layer separation, horseshoe vortex, corner vortex, tip vortex,

endwall crossflow, and passage vortex are secondary flow

components in the cascade. Many researchers investigated the

impact of three-dimensional blades and endwall boundary layer

separation as well as flow separation in corners of blade

passages on the development of secondary flows [1-3]. In order

to control the secondary flows, both passive and active methods

have been applied to reduce or overcome the effects of

secondary flows in axial compressors. It was found that the

passive control methods remain the preferable techniques

because of their simplicity and cost effectiveness [4-5].

Numerous types of passive flow control devices were

investigated such as slotted blading in linear cascades [6], vane

and plow vortex generators placed on several positions [7],

cavity as a control of shock wave interactions with the turbulent

boundary layer [8], low profile vortex generators to reduce the

boundary layer thickness [9], doublet vortex generators [10],

and wishbone vortex generator [11]. An excellent

comprehensive review of boundary-layer flow-separation

control by a passive method and their applications had been

compiled by [12-13]. There are numerous other reported

studies on the control of separation in turbulent boundary

layers using low profile vortex generators. In such devices,

the mechanism of flow control is to energize the low

momentum layers near a solid surface without adding extra

energy through the momentum transfer from the outer (free-

stream) flow to the near wall region. Yet, this leads to an

overwhelmed stronger adverse pressure gradient and hence

avoid or delay the flow separation. In case of turbulent flow


over flat plate, experimental results indicated that the vane and

wheeler type of vortex generators can efficiently reduce the

flow separation. Using the vortex generator height (h/δ) of 0.1

to 0.4 was fairly efficient with much reduce in the drag effect

[14]. It was reported that flow control by means of vane type

vortex generator arrays is robust with respect to changes in the

pressure gradient and changes of separation point [15]. In

addition, the van type with height (h/δ) of 0.8 attained the

largest pressure recovery [16]. McCormick [16] experimentally

compared between two passive methods for controlling shock

induced separation on a turbulent flat plate boundary layer. A

doublet wedge type vortex generator with h/δ=0.36 was used

versus passive cavity (porous wall with a shallow Cavity

underneath). He reported that the low profile vortex generators

were found to be significantly suppressing the shock induced

separation and improve the boundary layer characteristics

downstream the shock whereas the mass-averaged total

pressure loss increased. In case of turbulent flow over

backward facing ramp [18], wheeler doublet and wishbone type

vortex generators were used to control flow separation. They

concluded that both wheeler doublet and wishbone type

achieved the best effect in separation control when their heights

(h/δ) varied in the range of 0.1 to 0.2. Kerho [11] uses wishbone

VG to control the laminar separation, and he reported that

h/δ=0.3 caused the highest drag reduction during his

investigation. Diaa et al [19-20] have investigated the effect of

the curved side vortex generators as a control device with axial

compressor cascade, they reported a reduction of total pressure

loss of 20.7%.

Based on the above literature review, it is found that the

Wishbone and Doublet VGs are effectively used to eliminate

the separation and reduce the pressure loss on flat plates [11,

18]. Moreover, the influences of Wishbone and Doublet VGs on

axial compressors have not been investigated yet. Therefore, the

objective of the present study is to explore the effect of both

Doublet and Wishbone VGs on the boundary layer

characteristics and consequently the resulting secondary flow

losses, the total pressure loss, and the skin friction on the

endwall. This study will provide the researchers with the

opportunity to evaluate the visibility of using such

configurations as a boundary layer control devices to improve

the performance of axial compressors.

NUMERICAL APPROACH

AXIAL COMPRESSOR BLADE A transonic axial compressor cascade, that was designed by

MTU aero-engines and used by Hergt et.al [21], is used in this

study. The design parameters are summarized in Table 1.

COMPUTATIONAL DOMAIN The computational domain and its boundaries are shown

in Fig.1. Non-slip boundaries are considered at the bottom and

top endwalls and the blade sides, as well as vortex generator

surfaces. In addition, a periodic boundary condition is applied

at the computational domain sides. One pitchwise

computational domain is adopted where the blade is centered in

the middle of the domain. The vortex generator is placed on the

bottom endwall upstream the blade leading edge. The pressure

outlet boundary condition is defined at the outlet plane. The

fully developed flow is implemented at the inlet with an average

Mach number of 0.66 and inlet angle (β1) of 132° .

Table 1 Design operation parameters of the compressor

cascade.

Mach number at inlet M1 = 0.66

Inlet flow angle β1=132°

Turning angle Δβ = 38°

Stagger Angle βst =105.2°

Blade chord C = 40 mm

Blade span L = 40 mm

Pitch to chord ratio S/C = 0.55

Endwall boundary layer thickness δ = 4 mm

Figure1. Computational domain and its boundaries.

VORTEX GENERATORS Two types of vortex generators (doublet and wishbone) are

investigated as a boundary layer control device. Schematic

sketch of ddoublet and wishbone VGs are shown in Fig.2, and

their geometrical parameters are summarized in Table 2.

Figure 2. Schematic sketch of doublet (left) and wishbone

(right) vortex generators with geometrical parameters.

Table 2 Geometrical parameters of Doublet and Wishbone VGs.

VG h/δ e/h w/h t1/h t2/h

Doublet 0.4 5 6 ــــ ــــ

Wishbone 0.4 5 6 0.8 1.2


NUMERICAL SIMULATION To investigate the effect of inserting different types of

vortex generators on the boundary layer, the Reynolds-

average Navier–Stokes equations coupled with transitional

shear stress transformation (SST) turbulence model are

numerically solved using the commercial solver ANSYS

FLUENT. A three dimensional multiblock grid is constructed

using a structured mesh.

Solution free of grid dependency is attained by testing four

different mesh sizes ranging between 0.4 and 1.6 million cells

as shown in Fig.4. The Solution independent of the grid is

reached at a grid size of 0.8 million cells or more. Therefore,

the present simulation is performed using one million cells to

guarantee that the results are independent of the grid size along

with a reasonable computational time. In fact, many quantities

are investigated such as velocity, total pressure, and total

pressure loss coefficient. For the sake of brevity, only one figure

is included to clarify the grid independency results.

Figure 4. Variation of mass averaged velocity at exit plane

(located 40% of chord length behind trailing edge) with

different mesh size starting from 0.4 to 1.8 million cells.

The minimum cell height near the walls is adopted to have

y+<1, which is considered to capture and resolve the boundary

layer on the blade surfaces and enwalls. Mesh details for the

two types of VGs are shown in Fig.5.

VALIDATION The predicted numerical results are compared with the available

results reported by Hergt et.al [21]. Isentropic Mach number

distribution along the blade surfaces (suction and pressure) at

the midspan is plotted against the isentropic Mach number

calculated from the experimental work by Hergt [21] as shown

in fig. 6. The comparison shows a good agreement between

Hergt's and the present results.

Figure 5. Mesh details of the two vortex generator cases,

Doublet (Top), and wishbone (Bottom).

Figure 6. Mach isentropic distribution over the blade profile (at

midspan) of the present simulation and Hergt et. al[21].

RESULTS AND DISCUSSION In this section, the effect of the two types of VGs on the

vortex structure within the flow passage as well as the

separation lines distribution along suction blade surface as well

as the bottom endwall are presented and discussed.

Effect of vortex generators on the flow vorticity In order to track the changes of the vorticity magnitude by

using the vortex generators, four measurement planes are used

and placed at different positions along the blade. The four


planes are A, B, C and D placed at 20%, 40%, 60%, and 80% of

chord length respectively. The vorticity contours along the

blade chord are shown in fig. 7 for all studied cases.

First for base case (without VG) at plane A , as shown in

fig.7-a, The vorticity is noticed to have the highest value near

the top and bottom walls due to the endwall boundary layer.

Plane B shows a growth in the vorticity magnitude near the

blade walls and a slight increase in the vorticity near the corner

of the blade suction surface and both enwalls. Further

development in the vorticity is observed at suction surface in

plane C. This increase in the vorticity is considered to be an

indicator for the passage vortex formation and propagation

affecting the separation from the suction surface. Plane D shows

a reduction in the vorticity strength near the endwall with an

increase in the vorticity diffusion area. This occurs due to the

endwall boundary layer growth and the mixing of the low

momentum flow with the main passage flow.

When wishbone VG is inserted in fig.7-b, vorticity

contours suffer from some changes in the value and distribution

along monitored planes. At plane A, a spot area of high vorticity

is noticed above the observed vorticity in the base case. This

circular area indicates the suction side component of the

wishbone's generated vortex. For plane B, an increase in the

spot area of the high vorticity is noted which points to the

interaction between the generated and the passage vortices. At

plane C, a slight reduction in the generated vortex strength is

noted; along with a reduction in the vorticity magnitude near the

blade suction surface. This smear in the vortex strength

approves that the wishbone's generated vortex reduces the

passage vortex strength. Plane D shows a further stretching of

the generated vortex with lower vorticity magnitude, while the

occupation area of high vorticity near the bottom endwall and

the suction surface is slightly reduced. This refers to a further

mixing of the low momentum fluid of the endwall, and blade

suction surface boundary layer and the high momentum fluid.

When doublet VG is inserted in fig.7-c, plane A shows the

appearance of circular high vorticity area similar to that

generated with doublet VG. The circular area is more shifted

toward the mid flow passage due to the high deflection of the

generated vortex along the VG side and the double length in

streamwise direction of the VG. At plane B, the track area of

the vorticity is increased and becomes close to the suction

surface due to mixing of the low momentum fluid with high

momentum fluid supported with the cross flow from the

pressure to suction side. In plane C, more reduction in the

vorticity magnitude is registered. At the same plane, a reduction

in bottom endwall vorticity is noticed because of a continued

mixing. A slight deformation is noticed in the vorticity area

adjacent to the suction surface. By reaching plane D, vorticity at

the corner region between bottom endwall and suction surface

is diminished. In addition, a further reduction in vorticity

magnitude is observed at the bottom endwall. Figure 7. Vorticity magnitude contours at different planes A, B,

C, and D for the a-Base case (without VG), b-Wishbone VG,

and c-Doublet VG.


Effect of vortex generators on the streamlines across

blade Streamlines at the measurement planes A, B, C, and D are

used to track the propagation of the vortices inside the flow

passage between the blade suction and pressure surfaces as

shown in Fig.8.

For base case (without VG), as shown in fig.8-a, plane A

has no vortex formation. As the flow is propagating toward

plane B, the pressure side horse shoe vortex which is driven by

the pitchwise pressure gradient toward the suction surface, is

observed above the bottom endwall. Further movement toward

plane C, the lower passage vortex is appeared , it is also

noticeable, the appearance of the corner vortex in the junction

between the bottom endwall and the suction surface. At plane

D, the passage vortex is lifted from the bottom endwall toward

the midspan due to the cross flow effect. A magnified view near

the corner between suction surface and bottom endwall shows

the propagation of the corner vortex.

By inserting wishbone VG, the pressure side generated

vortex starts to appear near the bottom endwall at the periodic

boundary. This early appearance of pressure side vortex

provides more mixing at the bottom endwall boundary layer.

For plane B, the pressure side generated vortex continues its

propagation which is noticeable in the streamlines fig.8-b. At

plane C, the passage vortex appears near the lower suction

surface. It is slightly deflected toward the blade suction surface.

For plane D, lower passage vortex has been shifted slightly

toward the midspan. A magnified view of the corner vortex is

presented with using the wishbone VG. The view shows that no

significant change to the corner vortex is attained.

It is shown in fig.8-c that by using doublet VG, changes in

the streamlines are observed on the tracked panes. At plane A,

the pressure side generated vortex is more identifiable than that

of wishbone VG associated with a higher strength generated

vortex. Therefore, an energetic mixing of the bottom endwall

boundary layer with the main stream flow is achieved. It is also

noted that the generated vortex is slightly shifted toward the

periodic boundary (near the center of the flow passage). As the

flow propagates toward plane B, the generated vortex is

converted and propagated. This is causing additional mixing of

the bottom endwall boundary layer. At plane C, passage vortex

is noticed to be slightly deflected from the blade suction surface

toward the flow passage, due to the initially high strength of the

generated pressure side vortex. Another thing to be noticed at

this plane is the diminished corner vortex because of the

energized bottom endwall boundary layer. Moving to plane D,

a further deflection is caused for the passage vortex toward the

flow passage away from the blade suction surface.

Effect of vortex generators on limited suction surface

streamlines Figure.9 shows the surface streamlines at the blade suction

surface and the bottom endwall. The streamlines are used to

track the location and growth or decay of the separation lines

which represent the footsteps of the secondary flow structure.

Figure 8 streamlines at measurement planes A, B, C, and D for

a-Base, b-Wishbone, c-Doublet cases.


For base case, as shown in fig.9-a, node N at 35% of chord

length indicates the origin of the corner vortex. Suction side

corner vortex is separated from the blade suction surface at the

separation line SL, and re-attached to it at the attachment line

AL. Corner vortex footprint is also shown at the bottom endwall

where it is separated at SL2 and re-attached at AL2. In addition,

passage vortex which is driven by the pressure gradient between

the blade surfaces is identified by the separation line SL1 where

it is separated from the blade suction surface, and then re-

attaches to it at AL1. A circulation bubble CB is noticed at the

end of attachment line AL1. Cross flow and reverse flow

regions are identified by CF and RF respectively. The cross

flow is driven by the high pressure at the corner region toward

the midspan. While the adverse pressure gradient on the blade

suction surface causes the revers flow RF.

By inserting wishbone VG, as shown in fig.9-b ,node N is

moved downstream toward the trailing edge (at 40% of chord

length), because the generated vortex converted the horseshoe

vortex in the downstream direction. In addition, the separation

and the attachment lines of the corner vortex SL and AL have

slightly been moved toward the trailing edge. The movement of

the corner vortex over the bottom endwall is indicated by SL2

and AL2. The area between SL2 and AL2 is larger than the

same area at the base case, which indicates a growth in the

suction surface boundary layer at the lower corner

In other words, the generated vortex has not affected the

corner vortex; in contrast an increase in the corner vortex is

attained. For the passage vortex, a slight deflection in the

downstream direction is noticed, which could be identified by

the movement of SL1 and AL1 Also, a further movement of

circulation bubble CB in the downstream direction is noticed.

Consequently, the generated vortex causes a slight deflection of

the passage vortex in the downstream direction. Another

circulation bubble CB1 appears on the suction surface due to

the interaction between the cross flow driven by the adverse

pressure gradient at the lower corner and the streamwise flow.

Cross flow region CF is reduced in the downstream direction,

while the reverse flow region RF is shifted toward the midspan

leading to a probable increase in the total pressure loss.

By inserting doublet VG, as shown in fig.9-c node N is

moved towards the trailing edge at 55% of chord length. This

large movement due to the high shooting angle of the

horseshoe vortex introduced by the vortex generator. The

separation and reattachment lines of the corner vortex on the

suction surface (SL and AL) no longer exist. In addition, the

area between SL2 and AL2 is diminished, which indicates a

reduction in suction surface boundary layer growth. Therefore,

doublet VG increases the mixing of the endwall boundary layer

and reduces the corner separation. However, the area between

SL1 and AL1 (which indicates the passage vortex separation

and re-attachment lines) is increased and slightly shifted

towards the trailing edge leading to an increase in the total

pressure loss. No significant changes is noted in the cross flow

CF, reverse flow RF, and the circulation bubble CB1.

Figure 9 Streamlines on Bottom endwall and the Suction

surface for a-Base, b-Wishbone, and c-Doublet cases.


Effect of vortex generator on total pressure loss

coefficient Figure 10 shows the total pressure loss coefficient (TPLC)

contours at the exit plane (located at 40 % of chord length

downstream the trailing edge). TPLC is calculated from Eq. 1

where the subscript 1 indicates the inlet plane, and subscripts 2

indicated the exit plane. Inserting wishbone VG causes an

identifiable increase of total pressure loss in comparison with

base case. Mass averaged TPLC in case of wishbone VG is

increased by 4.3 % (as shown in fig.10-b) compared with the

base case. Similar effect on TPLC is noticed when using

doublet VG, total pressure loss is increased. The increase in

mass averaged TPLC caused by doublet VG reaches a value of

5.2 % over the base case as shown in fig.10-c. Therefore, using

doublet and wishbone VGs (as a boundary layer control device)

increases the total pressure losses in comparison with the base

case. This increase in TPLC for both VGs can be attributed to

the angle of injection of the generated vortices due to using

rounded nose instead of sharp one.

11

21 st

tt

pp

ppTPLC

(1)

Figure 10. Total pressure loss contours at exit plane for

a-Base, b-Wishbone, and c-Doublet cases.

Effect of vortex generators on friction coefficient Both vortex generators cause a significant effect on the skin

friction coefficient at the bottom endwall. Skin friction

coefficient is calculated from Equation 2.

2

2

1U

C wf

(2)

Skin friction coefficient (Cf) contours at bottom endwall

for Base, wishbone and doublet cases is shown in Fig.11.

Figure 11. Skin friction contours at bottom endwall for a-Base,

b-Wishbone, and c-Doublet cases.

From this figure, wishbone VG causes a significant

reduction in friction coefficient Cf in comparison with the Base

case, with a value of 32% as in fig.10-b. This reduction in

friction coefficient is increased by using Doublet VG to 46% as

shown in fig.10-c. This significant reduction in friction

coefficient stimulates further investigation to be conducted on

those two types of VGs.

FUTURE WORK The investigation will be extended by using different

geometrical ratios (for both doublet and wishbone vortex

generators) that probably causes a TPLC reduction

accompanied with aerodynamic enhancement for the

performance of the compressor cascade.

CONCLUSION Numerical simulations of a three-dimensional compressible

turbulent flow have been performed to explore the effect of

non-conventional vortex generators such as Wishbone and

Doublet as a boundary layer control devices. The investigation

is carried out using a transonic compressor cascade at the

design operation with inlet Mach number of 0.66. Numerical

simulation is executed using ANSYS FLUENT. The results

shows a moderate enhancement effect of the two vortex

generators on the cascade secondary flow structure. This

appears in shifting the passage vortex initiation towards the


trailing edge and reducing (Doublet VG) or shifting (Wishbone

VG) the corner vortex between suction and the bottom endwall

surfaces. In addition a significant reduction in friction

coefficient at the bottom endwall is achieved by wishbone (up

to 32%) and Doublet (up to 46%) vortex generators. However,

an increase in total pressure loss is attained by using the two

vortex generators with a percentage of 4.3 % with Wishbone,

and 5.2 % with Doublet type. More investigation is

recommended to study the effect of the various geometrical

parameters of the two types in order to reach the optimum

geometrical parameters that cause enhancement in the

aerodynamic structure of the axial compressor cascade with

considerable reduction in the total pressure loss.

NOMENCLATURE

Symbols

A,B,C,D =Measurement planes

AL =Attachment line

C =Chord

CB =Circulation bubble

CF =Cross flow

Cf =Skin friction coefficient

e =VG length

h = VG height

L =Span

M =Mach number

N =Separation node

P = Pressure (static or total)

RF =Reverse flow

S =Pitch

SL =Separation line

t =Thickness

TPLC =Total pressure loss coefficient

U = velocity magnitude

VG =Vortex generator

w =VG width

β =inlet flow angle

δ =Boundary layer thickness

ρ =Density

σ =solidity

τ =Wall shear stress

Subscripts

1 =inlet plane

2 =exit plane

is =isentropic

s =static pressure

st =stagger

t =total pressure

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