Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E6: Aerodynamics: Mathematical Models, Simulation and Shear Flow |
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Chair: Ricardo Vinuesa, Royal Institute of Technology (KTH) Room: B114 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E6.00001: High-order numerical simulations of the flow around wings at moderately high Reynolds number Ricardo Vinuesa, Prabal Negi, Seyed M. Hosseini, Ardeshir Hanifi, Dan S. Henningson, Philipp Schlatter The results of a DNS of the flow around a wing section represented by a NACA4412 profile, with $Re_{c} = 400,000$ and $5^{\circ}$ angle of attack, are presented in this study. The high-order spectral-element code Nek5000 was used for the computations. The Clauser pressure-gradient parameter $\beta$ ranges from $\simeq 0$ to $85$ on the suction side, and the maximum $Re_{\theta}$ and $Re_{\tau}$ values are around $2,800$ and $373$, respectively. The adversre pressure gradient (APG) on the suction side of the wing leads to a progressively increasing value of the inner peak in the tangential velocity fluctuations, as well as the development of an outer peak, which is also observed in the other components of the Reynolds-stress tensor. Close to the trailing edge, i.e., at $x/c \simeq 0.9$, the outer peak in the inner-scaled tangential velocity profile is larger than the inner peak. These effects are connected to the fact that the large-scale motions of the flow become energized due to the APG, as apparent from spanwise-premultiplied power spectral density plots. Preliminary comparisons between DNS and well-resolved LES data, based on a relaxation-term filtering approach, are also presented with the aim of further extending the Reynolds number to $Re_{c} \simeq 1,000,000$. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E6.00002: An EnKF-based Flow State Estimator for Airfoils at High Angles of Attack Andre Fernando de Castro da Silva, Tim Colonius Robust flow estimation from available measurements remains a major obstacle to successful flow control applications. Although several estimation methodologies have been developed in the past decades, the high dimensionality of fluid systems renders many of them computationally intractable. In this work, we employ the Ensemble Kalman Filter (EnKF) and the two-dimensional incompressible Navier-Stokes equations to estimate the state of the flow past a NACA 0009 airfoil at high angles of attack and moderate Reynolds number. The pressure distribution on the airfoil and the velocity field in the wake, both randomized by synthetic noise, are sampled as measurement data. In order to evaluate the relative importance of each sensor location to the estimate correction, their influence fields (also known as representers) are analyzed. The performance of the estimator is then assessed for different choices of ensemble size, noise levels, and number/location of sensors. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E6.00003: Mathematical Model for a Simplified Calculation of the Input Momentum Coefficient for AFC Purposes Damian Hirsch, Morteza Gharib Active Flow Control (AFC) is an emerging technology which aims at enhancing the aerodynamic performance of flight vehicles (i.e., to save fuel). A viable AFC system must consider the limited resources available on a plane for attaining performance goals. A higher performance goal (i.e., airplane incremental lift) demands a higher input fluidic requirement (i.e., mass flow rate). Therefore, the key requirement for a successful and practical design is to minimize power input while maximizing performance to achieve design targets. One of the most used design parameters is the input momentum coefficient $C_\mu$. The difficulty associated with $C_\mu$ lies in obtaining the parameters for its calculation. In the literature two main approaches can be found, which both have their own disadvantages (assumptions, difficult measurements). A new, much simpler calculation approach will be presented that is based on a mathematical model that can be applied to most jet designs (i.e., steady or sweeping jets). The model-incorporated assumptions will be justified theoretically as well as experimentally. Furthermore, the model’s capabilities are exploited to give new insight to the AFC technology and its physical limitations. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E6.00004: Experimental Study of Thin NACA Symmetric and Cambered Airfoils at Low Reynolds Numbers Vibhav Durgesh, Elifalet Garcia, Hamid Johari The low-Reynolds number performance of airfoils is intriguing due to the complex fluid dynamics phenomena associated with flow at these Reynolds numbers, like laminar separated flow, increased transition susceptibility, and the separated shear layer that undergoes a rapid transition to a turbulent flow. Therefore, the objective of this investigation was to experimentally study the aerodynamic performance of a thin symmetric airfoil (NACA-0012) and a cambered (NACA-6412) airfoil at low Reynolds numbers, and to identify the flow structures responsible for altering the aerodynamic performance. Lift and drag force measurements were performed for both airfoils along with flow visualization measurements for Reynolds numbers of 20,000, 30,000, 40,000, and 50,000 and angles of attack between $-8^o$ to $15^o$ with an increment of $1^o$. All the measurements for this study were performed in the water tunnel facility at California State University Northridge. A significant difference in the aerodynamic performance and flow behavior of the thin cambered airfoil is observed as compared to that of the thin symmetric airfoil. The presentation will discuss the correlation between observed flow structures and aerodynamic performance of both airfoils at low-Reynolds numbers. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E6.00005: Lift on a Steady Airfoil in Low Reynolds Number Shear Flow Patrick Hammer, Miguel Visbal, Ahmed Naguib, Manoochehr Koochesfahani Current understanding of airfoil aerodynamics is primarily based on a uniform freestream velocity approaching the airfoil, without consideration for possible presence of shear in the approach flow. Inviscid theory by Tsien (1943) shows that a symmetric airfoil at zero angle of attack experiences positive lift, i.e. a shift in the zero-lift angle of attack, in the presence of positive mean shear in the approach flow. In the current work, 2D computations are conducted on a steady NACA 0012 airfoil at a chord Reynolds number of \textit{Re} $=$ 12,000, at zero angle of attack. A uniform shear profile (i.e. a linear velocity variation) is used for the approach flow by modifying the FDL3DI Navier-Stokes solver (Visbal and Gaitonde, 1999). Interestingly, opposite to the inviscid prediction of Tsien (1943), the results for the airfoil at zero angle of attack show that the average lift is negative in the shear flow. The magnitude of this lift grows as the shear rate increases. Additional results are presented regarding the physics underlying the shear effect on lift. A companion experimental study is also given in a separate presentation. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E6.00006: Experiments on a Steady Low Reynolds Number Airfoil in a Shear Flow David Olson, Ahmed Naguib, Manoochehr Koochesfahani The aerodynamics of steady airfoils in uniform flow have received considerably more attention than that of an airfoil operating in a non-uniform flow. Inviscid theory by Tsien (1943) shows that an airfoil experiences a decrease in the zero lift angle of attack for a shear flow with uniform clockwise vorticity. The current work utilizes a shaped honeycomb technique to create a velocity profile with a large region of uniform shear in a water tunnel. Direct force measurements are implemented and validated using experiments on a circular cylinder and NACA 0012 in a uniform cross-flow. Results for a NACA 0012 airfoil with a chord Reynolds number of 1.2$\times$10$^4$ in a non-uniform approach flow are compared to concurrent CFD calculations (presented in a companion talk) showing an increase in the zero lift angle of attack; in contradiction with inviscid theory. The effect of shear on the mean lift coefficient over a wide range of angles of attack is also explored. [Preview Abstract] |
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