The schlieren image of the case with side walls permits the investigation of the flow field through the two window sections and above the side walls (Fig. 4-45). The cowl plate has been omitted for a better comparison to the reference case without side walls. The schlieren image indicates a shock wave due to the side wall installation, the first ramp shock mainly covered by the side wall, two separation shocks, one thick reattachment shock of the SWBLI as well as an expansion fan at the intake’s shoulder.
Fig. 4-45: Schlieren image with installed side walls, Cond. I, TW = 300 K
The visible layer upstream of the interaction is thicker than the visible layer of the refer- ence case in Fig. 4-27 so that the height of the zoom section is increased to 10 mm. In Fig. 4-27 the visible layer represents the boundary layer, but in the image of the side wall case the visible layer is the superposition of the boundary layer and the corner flow (chapter 2.5.3) thus the height of the corner flow is more than the double of the boundary layer height. Moreover, the first separation shock is hardly visible so that it is indicated by a white line. For a decreased density gradient range of the schlieren image, the first separation shock is displayed (Fig. 4-46) so that zoomed images with a decreased measurement range of the density gradient allow the determination of the separation point. The second separation shock is close to the kink and mainly hidden by the corner flow. Downstream of the reat- tachment shock no clear discontinuity line is visible due to the missing of a defined triple point of the separation, the reattachment and the second ramp shock.
To explain the occurrence of two separation shocks of which one is hardly visible, the IR image (Fig. 4-29) is employed. Upstream of the interaction the heating due to the corner flow is clearly observed. As for the case without side walls the separation line is defined by the temperature decrease and therewith has a kind of tongue form. The parts of the separa- tion line being further downstream are in the regions of the corner flow which transports high energetic flow to the wall. Hence, the corner flow causes the increased heat load ob- served in the IR image. Since the energized boundary layer is able to better withstand the upstream acting pressure, the separation size is decreased in the region of the corner flow. The separation position in the model’s midspan is increased compared to the reference case because the side flow is prevented by the side wall installation. Concluding, in the lateral direction the first ramp can be categorized into the parts influenced by the corner flow and a midspan part with corresponding separation positions and separation shocks. The schlieren
image compresses this information over the flow field depth thus the tongue form of the separation line and therewith the corresponding separation shock form appear as two shocks. The classification of the first ramp flow in such parts simplifies the understanding, but as the separation line reveals, no sharp borderline exists between those parts. Nevertheless, if this categorization is used for the second ramp flow, two reattachment shocks in the schlieren image are expected. Since the variation of the reattachment line with respect to the lateral direction is small, both expected shocks appear as one thick reattachment shock. Further conclusions with regard to the schlieren image are drawn after the discussion of the pressure distribution.
Fig. 4-46: Zoomed image of Fig. 4-45: SWBLI
The pressure distribution along the midspan of the model with side walls (WSW) repre- sents the midspan flow with a larger separation size than the one obtained for the reference case (Fig. 4-47). Both cases reach a similar plateau pressure cp,II, but the pressure rise is
slightly higher for the case with side walls. Such increased pressure rise is the result of the prevented side flow as well as the additional compression due to the boundary layers devel- oping on the inner sides of the side walls. This additional compression can be observed by the slight pressure increase even upstream of the separation position. The numerical solution representing the two-dimensional case indicates a smaller separation size than observed for the midspan part of the side wall case. The reattachment pressure for the side wall case is drastically increased compared to the other distributions shown. Moreover, the prevented side flow can also be observed by the comparison of the Goertler vortices’ footprints in the IR images (Fig. 4-28 and Fig. 4-29) which are oriented in streamwise direction for the in- stalled side walls.
s/L
1[-]
c
p[-
]
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0.4 0.5 0.6 Exp. WSW Exp. Ref. caseCFD c
p,III
cp,II cp,I
Ref. case & WSW
WSW Ref. case
Fig. 4-47: Pressure distributions with and without side walls as well as numerical simulation, Cond. I, dashed sep. and rea. positons from schlieren images
For the case with side walls the pressure rise at separation is employed for further inter- pretation of the schlieren image. Since this pressure rise measured in the model’s midspan part extents over a longer distance in downstream direction, such pressure rise is achieved by a weaker shock wave compared to the reference case. This weaker shock wave features a smaller density gradient thus the separation shock is hardly visible in the schlieren image. The light deflection captured by the schlieren technique depends, besides of the density gradient, also on the integration length being the lateral extent of the density gradient in the flow field. Such extent is decreased for the case with side walls as only the midspan part acts as integration length compared to the whole model width acting as integration length for the reference case. Since the schlieren image displays a strong and discrete density gradient upstream of the kink, the pressure rise in the parts influenced by the corner flow is accom- plished by oblique shock waves which result in the pressure rising to the plateau pressure in a shorter distance than in the midspan flow.
The Stanton number distribution determined by the thermocouples also indicates a larger separation length in the midspan of the model (Fig. 4-48) and agrees with the separation positions obtained by the schlieren images. The most upstream thermocouple (s/L1 = -0.58)
upstream of separation (s/L1 = -0.27 and -0.23) indicate lower heat fluxes due to the pre-
vented side flow. Therefore, the side wall installation leads to a thicker boundary layer in the midspan of the model and this thicker boundary layer insulates the wall better from the high enthalpy outer flow and thereby reduces the heat flux. Downstream of the reattachment region the wall pressure is increased due to the larger pressure rise of the separation and the reattachment shock compared to the pressure rise of the inviscid ramp shock only. Such increased wall pressure corresponds to a higher static temperature at the boundary layer edge which has an increasing effect on the heat load. Besides the increased boundary layer thickness the increased separation size spreading the reattachment process over a wider distance has also a reducing effect on the heat load. Concluding, the increased compression of the flow is superior compared to the other influences so that the reattachment heat load is increased. In the most downstream portion of the second ramp (s/L1 = 0.35 to 0.44) the IR
line scan indicates that the heat flux still increases and even exceeds the estimated turbulent level in the region downstream of the Goertler vortices. The exceeding of the turbulent heat flux level is also the result of the increased boundary layer edge temperature. In comparison all runs without side wall indicate that none of them exceeds the turbulent estimate (Fig. 4-35). The increased heat load leads to higher temperatures measured by the IR camera so that the displayed temperature range for the IR image of the case with side walls (Fig. 4-29) is raised compared to the IR image of the reference case (Fig. 4-28).
s/L
1[-]
C
H[-
]
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 10-4 10-3 10-2 Exp. WSW Exp. WSW IR y = 17 mm Exp. Ref. caseWSW Lam. B.l.
Lam. B.l. Turb. B.l.
Ref. case & WSW Ref. case
Fig. 4-48: Stanton number distributions with and without sidewalls, Cond. I, TW = 300 K, dashed sepa-
For the case with side walls the thermocouple measurements and the IR line scan show different separation positions which are indicated by the rapid heat flux decrease. The line scan’s position (y/B = 0.17) is shifted from the midspan thus indicates a separation position between the midspan flow and the corner influenced flow (Fig. 4-29). The reattachment line is marked by the upstream edge of the Goertler vortices’ footprints and is representative for the form of the separation line. The Goertler vortices’ distribution appears to be more regu- lar compared to the reference case. Also the number of vortices’ footprints is increased thus the vortex’s diameter is nearly unchanged. The flow field downstream of the Goertler vor- tices’ breakdown shows the same behaviour as described for the reference case.
The finding of the separation line’s tongue form is also confirmed by the lateral pressure and Stanton number distributions (Fig. 4-49). The most upstream pressure distributions of the two cases (s/L1 = -0.461) indicate no deviation in the midspan of the model (y/B 0.2)
as well as a slight decrease towards the model sides for the reference case and a slight in- crease for the side wall case. With regard to the lateral pressure distribution upstream of the kink (s/L1 = -0.13) the outer measurements of the side wall case (y/B = 0.3 and 0.4) are
located upstream of the separation line whereas the inner measurements (y/B = 0.0 and 0.1) are located within the separation bubble. Therefore, the inner ones agree with the plateau pressure coefficient cp,II and the outer ones are unchanged compared to the measurements
upstream of separation ((s/L1 = -0.461). For the case with side walls the most upstream
Stanton number distribution (s/L1 = -0.561) in lateral direction measured a heat flux de-
crease from the midspan towards the side wall. According to the IR image (Fig. 4-29) the corner flow increases the heat load locally which can not be observed due to the limited spatial resolution of the thermocouples in lateral direction and the malfunction of the ther- mocouple at the position y/B = 0.3 which therefore is not included at this position in Fig. 4-49 (right). The lateral Stanton number distribution upstream of the kink (s/L1 = -0.081)
crosses the separation line so that the inner thermocouples (y/B = 0.0 and 0.2) are located within the separation bubble which is indicated by the heat flux decrease due to the laminar separation. The outer thermocouples (y/B = 0.3 and 0.4) measured an even higher heat flux than the reference case which is caused by the corner flow and can also be observed in the IR image (Fig. 4-29). This spreading of the corner flow from the side walls towards the centre line with increased running length can be described similar to the side flow spreading angle sketched in Fig. 4-42. The corresponding corner flow spreading angle is determined between 4.6° and 4.8°. This range of the corner flow spreading angle is determined with respect to the fact that the angle has to be smaller than 4.8° because the outmost thermocou- ple measurement in the most upstream position (s/L1 = -0.561, y/B = 0.4) is not affected by
the corner flow. On the other hand the angle has to be larger than 4.6° to reach the second thermocouple counted from the side wall upstream of the kink (s/L1 = -0.081, y/B = 0.3).
y/B [-] CH [- ] 0 0.1 0.2 0.3 0.4 10-4 10-3 10-2 s/L1= -0.561 s/L1= -0.081 s/L1= 0.325 WSW s/L1= -0.561 WSW s/L1= -0.081 WSW s/L1= 0.325 y/B [-] cp [- ] 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0.5 0.6 s/L1= -0.46 s/L1= -0.13 s/L1= 0.24 WSW s/L1= -0.46 WSW s/L1= -0.13 WSW s/L1= 0.24 cp,II cp,III cp,I
Fig. 4-49: Pressure (left) and Stanton number distribution (right) in lateral direction for the reference case and the intake model with installed side walls (WSW), TW = 300 K
The lateral pressure and Stanton number distributions downstream of the interaction confirm the finding of an increased flow compression due to the side wall installation with corresponding heat load increase. The pressure distributions of the case with and the case without sidewalls demonstrate that the compression decays towards the sides, but the de- crease for the side wall case is more pronounced. Such pronounced pressure decrease is the result of the increased compression due to the displacement effect caused by the corner flow. The Stanton number distribution of the side wall case for this position (s/L1 = 0.325)
shows an increased heat flux level compared to the reference case in the model’s midspan (y/B 0.3) with a rapid decrease towards the side walls. This severe decrease leads to heat flux levels lower than upstream of the interaction and is the outcome of the complex corner flow interacting with the SWBLI in compression corner’s kink.