Satya P. Ojha

Turbulence statistics of flow over isolated scalene and isosceles triangular-shaped bedforms

The objective of this research is to examine the turbulent flow characteristics over artificial waveform structures and to compare turbulence between two types of isolated geometries. Measurement of turbulent velocity components have been performed over the respective structures of equal crest height and length at the Fluvial Mechanics Laboratory (FML) of the Indian Statistical Institute, Calcutta. The velocity data were analyzed to determine the relative importance of mean flows, Reynolds stresses and the contributions of burst-sweep cycles. The difference in the sizes of the separation zone for these two cases makes significant differences in the mean flow and turbulence. The motivation of this study is to identify the spatial changes of flow and turbulent events over these structures, and to gain better insight in the flow physics, which are different conditions for the process of sediment transport.

Land Surface Temperature from INSAT‐3D Imager Data: Retrieval and Assimilation in NWP Model

A new algorithm is developed for retrieving the land surface temperature (LST) from the imager radiance observations on board geostationary operational Indian National Satellite (INSAT-3D). The algorithm is developed using the two thermal infrared channels (TIR1 10.3–11.3 µm and TIR2 11.5–12.5 µm) via genetic algorithm (GA). The transfer function that relates LST and thermal radiances is developed using radiative transfer model simulated database. The developed algorithm has been applied on the INSAT-3D observed radiances, and LST retrieved from the developed algorithm has been validated with Moderate Resolution Imaging Spectroradiometer land surface temperature (LST) product. The developed algorithm demonstrates a good accuracy, without significant bias and standard deviations of 1.78 K and 1.41 K during daytime and nighttime, respectively. The newly proposed algorithm performs better than the operational algorithm used for LST retrieval from INSAT-3D satellite. Further, a set of data assimilation experiments is conducted with the Weather Research and Forecasting (WRF) model to assess the impact of INSAT-3D LST on model forecast skill over the Indian region. The assimilation experiments demonstrated a positive impact of the assimilated INSAT-3D LST, particularly on the lower tropospheric temperature and moisture forecasts. The temperature and moisture forecast errors are reduced (as large as 8–10%) with the assimilation of INSAT-3D LST, when compared to forecasts that were obtained without the assimilation of INSAT-3D LST. Results of the additional experiments of comparative performance of two LST products, retrieved from operational and newly proposed algorithms, indicate that the impact of INSAT-3D LST retrieved using newly proposed algorithm is significantly larger compared to the impact of INSAT-3D LST retrieved using operational algorithm.

Suspension concentration due to combined wave-current flows over waveforms

Abstract This paper presents the results of an experimental study carried out in a laboratory to investigate the sediment suspension due to combined wave-current flows over asymmetric dunes placed successively at the flume. The flume base was made by fixing 12 artificial asymmetric dunes. Experiments were performed with current only and combined wave-current flows over the series of bottom-waves under different frequencies of surface wave in sediment-laden flow for a fixed Reynolds number. The ninth dune was selected in this study, because the flow reached to a quasi–steady–state after the seventh dune. The eddy viscosity along the dune was found to be modulated due to formation of vortices on the dune flanks with the superposition of surface waves, but changes in vertical suspension concentration were rather weak with respect to locations as well as wave frequencies. The conventional diffusion equation was employed to estimate the vertical concentration distribution along waveforms with modified eddy viscosity for different frequency of waves. The results obtained using the modified Rouse equation compare quite well with the experimental observations. This model can address the suspension concentration that occurs in combined wave-current sediment-laden flow over waveform structures in real situation.

CONTRIBUTIONS OF BURST-SWEEP CYCLES TO THE REYNOLDS SHEAR STRESS OVER THE WAVEFORM STRUCTURES

ABSTRACT The purpose of the present paper is to investigate the contributions of turbulent events to the total Reynolds shear stress over artificial waveforms and to make a comparative study of turbulence between two types of isolated wave geometries: the scalene triangular shape (STS) and the isosceles triangular shape (ITS). All three components of velocity with fluctuations have been measured using 3-D Micro- Acoustic Doppler Velocimeter (ADV) at the Indian Statistical Institutes (ISI) flume, Calcutta. The motivation of this study is to determine the spatial changes of flow and turbulent events, and to gain better understanding of the physics of flow, which are responsible for the transport of sediment.