Discrete-vortex simulation of pulsating flow on a turbulent leading-edge separation bubble

Studies are made of the turbulent separation bubble in a two-dimensional semi-infinite blunt plate aligned to a uniform free stream with a pulsating component. The discrete-vortex method is applied to simulate this flow situation because this approach is effective for representing the unsteady motions of the turbulent shear layer and the effect of viscosity near the solid surface. The numerical simulation provides reasonable predictions when compared with the experimental results. A particular frequency with a minimum reattachment is related to the drag reduction. The most effective frequency is dependent on the amplified shedding frequency. The turbulent flow structure is scrutinized. This includes the time-mean and fluctuations of the velocity and the surface pressure, together with correlations between the fluctuating components. A comparison between the pulsating flow and the non-pulsating flow at the particular frequency of the minimum reattachment length of the separation bubble suggests that the large-scale vortical structure is associated with the shedding frequency and the flow instabilities

Comparison of very-large-scale motions of turbulent pipe and boundary layer simulations

A direct numerical simulation of a fully developed turbulent pipe flow was performed to investigate the similarities and differences of very-large-scale motions (VLSMs) to those of turbulent boundary layer (TBL) flows. The Reynolds number was set to ReD = 35 000, and the computational domain was 30 pipe radii in length. Inspection of instantaneous fields, streamwise two-point correlations, and population trends of the momentum regions showed that the streamwise length of the structures in the pipe flow grew continuously beyond the log layer (y/?? 3??), and the maximum length of the VLSMs increased up to ~30??. Such differences between the TBL and pipe flows arose due to the entrainment of large plumes of the intermittent potential flow in the TBL, creating break-down of the streamwise coherence of the structures above the log layer with the strong swirling strength and Reynolds shear stress. The average streamwise length scale of the pipe flow was approximately 1.5-3.0 times larger than that of the TBL through the log and wake regions. The maximum contribution of the structures to the Reynolds shear stress was observed at approximately 6?? in length, whereas that of the TBL was at 1??-2??, indicating a higher contribution of the VLSMs to the Reynolds shear stress in the pipe flow than in the TBL flow.open1

Migration of Elastic Capsules by an Optical Force in a Uniform flow

AbstractThe behavior of an elastic capsule by an optical force in a uniform flow is examined by using the penalty immersed boundary method. The elastic capsule is subjected to the laser beam with Gaussian distribution in the perpendicular direction to the fluid flow. The elastic capsule migrated by the optical force along the direction of the laser beam propagation, and the migration distance is dependent on its properties. The oblate capsule with b/a = 0.5 obeying the neo-Hookean constitutive law is first considered, and the effects of the surface Young's modulus and the initial inclination angle on the migration distance are studied. The migration distance of the oblate capsule is increased as the surface Young's modulus increases, and the non-inclined oblate capsule is more migrated than the differently inclined capsules. Then the spherical, oblate, and biconcave capsules obeying the Skalak constitutive law are considered. A comparison of the trajectories of the capsules indicates that the migration of the spherical capsule is the largest. Unlike the oblate capsule, the non-inclined biconcave capsule is less migrated than other inclination angles due to its initial shape

Travelling Surface Acoustic Waves Microfluidics

AbstractIn this paper, we demonstrate the working principle of travelling surface acoustic waves (TSAWs) in a microfluidic system. The TSAWs were incorporated to separate polystyrene (PS) particles of variable diameters and perform controlled mixing of different chemicals for concentration gradient generation, both inside a polydimethylsiloxane (PDMS) microfluidic channel. The TSAWs generated an acoustic streaming flow (ASF) upon coupling with a liquid and exerted an acoustic radiation force (ARF) on the suspended particles. The ARF was theoretically estimated for PS microspheres suspended in water, and conditions for ARF dominance over ASF or vice versa were identified. Recently reported TSAW-based PS particles separation and gradient generation results by our group are summarized here

Direct numerical simulation of a 30R long turbulent pipe flow at Re=3008

A direct numerical simulation of a turbulent pipe flow at a high Reynolds number of Re-tau = 3008 over a long axial domain length (30R) was performed. The stream-wise mean velocity followed the power law in the overlap region (y(+) = 90-300; y/R = 0.03-0.1) based on the power law indicator function. The scale separation of the Reynolds shear stresses into two components of small-and large-scale motions (LSMs) revealed that the LSMs in the outer region played an important role in constructing the constant-stress layer and the mean velocity. In the pre-multiplied energy spectra of the streamwise velocity fluctuations, the bimodal distribution was observed at both short and long wavelengths. The k(x)(-1) region associated with the attached eddies appeared in lambda(x)/R = 2-5 and lambda(x)/y = 18-160 at y(+) = 90-300, where the power law was established in the same region. The k(z)(-1) region also appeared in lambda(z)/R = 0.3-0.6 at y(+) = 3 and 150. Linear growth of small-scale energy to large-scale energy induced the k(x)(-1) region at high Reynolds numbers, resulting in a large population of the LSMs. This result supported the origin of very-large-scale motions in the pseudo-streamwise alignment of the LSMs. In the pre-multiplied energy spectra of the Reynolds shear stress, the bimodal distribution was observed without the k(x)(-1) region.open