Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


mwdissertation.pdf (19.45 MB)

ETD Abstract Container

Abstract Header

On Unsteadiness in 2-D and 3-D Shock Wave/Turbulent Boundary Layer Interactions

Waindim, Mbu
http://rave.ohiolink.edu/etdc/view?acc_num=osu1511734224701396

Abstract Details

2017, Doctor of Philosophy, Ohio State University, Aero/Astro Engineering.
Shock-boundary layer interactions (SBLIs) are ubiquitous occurrences in supersonic and hypersonic vehicles and have the tendency to inhibit their structural and aerodynamic performance. For example, in the inlets and isolators of such vehicles, the shock wave generated by one surface interacts with the boundary layer on an adjacent one. They are also present on the exterior of the vehicles, e.g. at the fuselage/vertical stabilizer junctions. These interactions cause unsteady separation, resulting in reduced air in-take efficiency, or unstart in extreme cases; unsteady vortex shedding which yields undesirable broadband noise; and significant pressure fluctuations which compromise the structural integrity of the vehicle and which can lead to loss of control authority. Mitigating these issues is therefore an important part of optimizing aerodynamic and structural design of high speed vehicles. The first step in this respect is obtaining a better understanding of the interaction unsteadiness. Nominally 2-D interactions have been studied extensively and have identified low-frequency shock motions which lead to undesirable pressure loads. The particular frequencies associated with the motions have been characterized using time resolved experiments and computations, and shown to depend on the mean size of the separation. The physical processes responsible for these frequencies are however still under investigation and the physical relationship between the shock motions and pulsations of the separation bubble remains obscure. For flow fields where the shock is swept, a complex 3-D interaction is encountered whose unsteady features are even less well understood. The mean structure of these 3-D interactions has been obtained experimentally and using RANS simulations, and shown to be profoundly different from the 2-D flow field indicating that progress in understanding 2-D interactions cannot be directly translated to 3-D. Specifically, there is no recirculating region in the 3-D interaction contrasting with the 2-D case where a closed separation bubble essentially drives the dynamics. This effort seeks to understand the unsteadiness in such 3-D interactions. Although Large Eddy Simulations (LES) are invaluable for characterizing time evolving features, they are prohibitively expensive for swept interactions. A stability analysis based method, the Mean Flow Perturbation (MFP) technique appears as a better alternative to LES in terms of computational cost. It involves tracking the evolution of disturbances through the interaction in space and time to identify the low frequency and stability properties of the chaotic 3-D dynamical system. A systematic verification and validation of the technique is presented to provide a solid basis for its employment. By implementing the technique on canonical problems with similar properties as the swept interaction, its applicability for flows with strong gradients and viscous effects is established. The method is then implemented on the simpler 2-D interaction with two specific goals: to ensure that MFP accurately captures well known unsteady features of the flow and to learn relevant lessons and establish best practices for SBLI application. By definition, the implementation of MFP requires the knowledge of a mean base state. For the nominally 2-D interactions, the mean of a previously obtained LES is used as the basis. In addition to providing the input to the MFP technique, the LES is used as a testbed to: (i) Improve effectiveness of a technique for generating spatially developing supersonic turbulent boundary layers (TBLs) as correctly characterizing the inflow boundary condition is a critical component of simulating SBLIs. (ii) Generate insight on possible physical mechanisms responsible for the low frequency unsteadiness in 2-D by exploiting asymmetry in the shock motions. Besides the LES supplied mean base state, the MFP technique is also implemented with a RANS supplied base flow. No significant differences in the results are observed indicating a relative independence of the MFP technique to the base flow generation procedure. Due to the highly prohibitive nature of implementing LES for 3-D interactions, the above observation acts as a substantial endorsement for using RANS generated base state for studying 3-D interactions with the MFP technique. It also provides specific insight into ways to correctly implement the technique when the base flow comes from RANS. Finally, the dynamics of the swept interaction are explored to characterize its unsteady and stability features. Here the basic state is obtained by tailoring RANS calculations to experiments at Florida State University, allowing the separation and other relevant mean features to be accurately captured. The flow field obtained by perturbing this mean is post processed to identify the length and time scales relevant to the flow field. This effort accomplishes three things: (i) a tripping technique is generated to efficiently specify turbulent boundary layer appropriate for use as the inflow condition, (ii) LES of 2-D interactions are obtained and analyzed to identify the cause of energetic low frequency and (iii) the relevant frequencies in the swept interaction are characterized using MFP and the stability properties of the interaction are identified, distinguishing its dynamics from the 2-D. Each step involves a myriad of statistical tools that can be adapted for other applications. The findings of each effort are presented. It is found that the effectiveness of tripping a laminar boundary layer to yield its turbulent counterpart is dependent on a range of factors: (i) grid resolution, (ii) strength of the force associated with the trip, (iii) wall thermal condition near the trip and (iv) Mach number. By characterizing the stability of the boundary layer in the trip region, a precursor for transition to turbulence is identified. This makes the method efficient for generating turbulent boundary layers at alternative desired flow conditions (Mach and Reynolds numbers) as appropriate trip parameters can be obtained a priori. Statistical analyses of the shock motions in 2-D interactions reveal an asymmetry whose quality appears to be dependent on the separation bubble size. For massive separation bubbles, where the shear layer is detached, the collapse phase is a rapid process, possibly owing to the ease of formation of Kelvin-Helmholtz (K-H) structures which convect mass and momentum out of the bubble, leading to its collapse. For moderate separation bubbles, the collapse phase is instead the slower process. It is found that the quality of the asymmetry is linked to the linearly stable position of the shock, which has implications for control. In addition, there is evidence of modulation of the frequencies associated with K-H shedding by the low frequencies characteristic of the shock motions, establishing a link between the two physical phenomena and further reinforcing the role of eddies in bubble collapse. The most substantial outcome of this work is the insight obtained regarding the dynamics of the swept interaction. The results show that: (i) The shock generated by the fin is anchored unlike the oscillating reflected shock in 2-D; consequently, the low frequencies observed in 2-D are not present in this swept interaction. (ii) A convective inviscid instability is identified at a frequency an order of magnitude higher than the characteristic frequency of shock motions in 2-D. It is shown to be a consequence of the crossflow and analogous to the mid frequencies associated with K-H shedding in the 2-D interaction. (iii) The absolute instability observed in 2-D does not persist here as the absence of a closed separation reduces the interaction’s ability to perpetually self-sustain introduced perturbations.
Datta Gaitonde (Advisor)
Jen-Ping Chen (Committee Member)
Jack McNamara (Committee Member)
Mo Samimy (Committee Member)
199 p.
Aerospace Engineering; Fluid Dynamics
shock waves; fluid dynamics; turbulence; boundary layers; computational fluid dynamics; CFD; unsteadiness; SBLI; dynamic mode decomposition; DMD; empirical mode decomposition; EMD; stability analysis

Recommended Citations

Citations

  • Waindim, M. (2017). On Unsteadiness in 2-D and 3-D Shock Wave/Turbulent Boundary Layer Interactions [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1511734224701396

    APA Style (7th edition)

  • Waindim, Mbu. On Unsteadiness in 2-D and 3-D Shock Wave/Turbulent Boundary Layer Interactions. 2017. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1511734224701396.

    MLA Style (8th edition)

  • Waindim, Mbu. "On Unsteadiness in 2-D and 3-D Shock Wave/Turbulent Boundary Layer Interactions." Doctoral dissertation, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1511734224701396

    Chicago Manual of Style (17th edition)

osu1511734224701396
231
© 2017, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.