1. Supersonic Inlets - Oblique Shocks -1 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Supersonic (Engine) Inlets For air-breathing engines on supersonic vehicles, usually want to slow flow down to subsonic speeds inside engine –need diffuser (M>1  M<1) for engine inlet –exception: supersonic combustion (e.g., SCRAM jets) Goal –lowest p o loss (highest thrust) given flight M mass flow rate requirement (thrust) stable operation (nothing drastic for small changes in flight conditions)

2. Supersonic Inlets - Oblique Shocks -2 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 CD Inlet Can get close to isentropic flow –lowest potential p o loss –starting problem (like supersonic wind tunnel, have to swallow shock) –most cases requires variable area throat (heavy, complex) –stability problem: if “shock” leaves throat, can exit engine lowering mass flow (higher A 2 * ) Not typically used M>1 M<1

3. Supersonic Inlets - Oblique Shocks -3 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Shock Inlets Normal shock diffuser –simple, diverging section preceded by normal shock –highest p o loss (strong shock) M>1 M<1 Spilled Air Normal Shock Bleed Air Oblique Shocks Normal Shock Spilled Air BypassAir Subsonic Diffuser Oblique Shock Diffuser –oblique shock(s) followed by normal shock at (or inside) inlet to subsonic diffuser –lower p o loss –works for range of M

4. Supersonic Inlets - Oblique Shocks -4 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Example: Normal v. Oblique Diffusers Given: You need to pick a diffuser for Mach 2 flight conditions. Your choices are a normal shock diffuser and 2 different oblique shock diffusers Find: Stagnation pressure loss for each (p o,final /p o,initial ) Assume: air is TPG/CPG with  =1.4, steady, adiabatic, no work, inviscid except for shock,…. M=2 20° M=2 10° 5°5° M=2 p o,initia l p o,final p o,initia l p o,final p o,initia l

5. Supersonic Inlets - Oblique Shocks -5 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Solution: Normal v. Oblique Diffusers Analysis: –Normal Shock Diffuser M 1 =2 p o1 M 2, p o2 Table B.1 or VII.11,13 –Single Oblique Shock (+Normal) 20° M 1 =2 M 2, p o,2 p o1 M 3, p o3 VII.26 or C.1 B.1 or VII.11,13 normal shock inlet - 28% p o loss oblique shock inlet - 13.3% p o !!!

6. Supersonic Inlets - Oblique Shocks -6 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Solution: Normal v. Oblique Diffusers Analysis: (con’t) VII.26 or C.1 B.1 or VII.11,13 –Two 5° Turns (+Normal Shock) M 2, p o,2 p o1 M 3, p o3 10° 5°5° M=2 VII.26orC.1 B.1 or VII.11,13 double obl. shock inlet ~ 12.7% p o loss

7. Supersonic Inlets - Oblique Shocks -7 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Double Oblique Inlet - Advantages So for M=2, same total turning angle (10°) –two oblique shocks slightly better than one (12.7% v. 13.3% p o loss ) –significant improvement over normal shock alone (28% p o loss) Oblique shock diffuser with two 10° turns (total  of 20°) even better –only 4.3% p o loss (solution shown on next slide) –so larger overall deflection can give better p o Stagnation pressure advantages of using multiple oblique shocks increase with higher M

8. Supersonic Inlets - Oblique Shocks -8 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Double Oblique Shock Diffuser Two 10° Turns (+Normal Shock) M 2, p o,2 p o1 M 3, p o3 from previous results for 10° turn B.1 or VII.11,13 20° 10° M=2 VII.26 or C.1 B.1 or VII.11,13

9. Supersonic Inlets - Oblique Shocks -9 School of Aerospace Engineering Copyright © 2001 by Jerry M. Seitzman. All rights reserved. AE3450 Double Oblique Inlet - Disadvantages Flow separation –since p increases across oblique shocks, the flow sees adverse p gradient –so more or bigger oblique shocks (or longer external ramp), the greater the chance the boundary layer will separate  major change in flowfield, large losses (p o and mass flowrate) Internal Turn Angle –larger total external turn angle showed less p o loss –larger external flow turning requires larger inlet –also requires larger internal flow turning to get flow back to horizontal Bleed Air Oblique Shocks Normal Shock Spilled Air BypassAir Subsonic Diffuser Boundary Layer