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In this paper, we perform experiments on the time-space characteristics of internal waves generated by horizontally towed bodies with three aspect ratios in a stratified fluid with a halocline. By the real-time measurements of conductivity probe arrays which are arranged symmetrically in the transverse section of the stratified fluid tank, it is shown that the transition between the body-generated internal wave and the wake-generated internal wave is related to a critical Froude number Frc, which is linearly dependent on the aspect ratio. For FrFrc, the correlation velocities of internal waves are consistent with the towing speeds of the towed bodies, indicating that such internal waves in this range are dominated by the body-forced effect. The heights of such body-generated internal waves first increase with the increase of Fr until Fr reaches a certain value of Frp, which is also linearly dependent on the aspect ratio, and then decrease. For FrFrc, the correlation velocities of internal waves are noticeably lower than the towing speeds, indicating that such internal waves in this range are dominated by the wake-forced effect, and that the Froude numbers with respect to the correlation velocities of such internal waves vary in a range from 0.43 to 1.18. The heights of such wake-generated internal waves nearly linearly increase with Fr increasing regardless of the aspect ratio. Moreover, the patterns of body-generated waves are symmetric, while the patterns of wake-generated waves are not symmetric. Based on the experimental results and the equivalent source method which has been proposed to simulate the internal waves generated by a towed sphere, a new equivalent source method is developed to calculate the internal waves generated by towed slender bodies. For the body-generated waves, the method of designing the speed, length and diameter of the equivalent source is proposed. The symmetrical and anti-symmetrical equivalent source and their speed and size are also proposed for the wake-generated waves. The numerical results are in good accordance with the experimental results in the heights and patterns of waves, indicating that such a theoretical method and its parameter settings are reasonable and effective.

A theoretical model for interaction of a submerged moving body with the conjugate flow in a three-layer fluid is proposed to depict the internal flat solitary wave, which is observed in experiments conducted by the present authors. A set of coupled nonlinear algebraic equations is derived for the interfacial displacements. The numerical results indicate that (a) the conjugate flow due to a two-dimensional body moving at the bottom possesses an apparent behaviour with two convex interfaces; (b) the solution satisfying the existence criterion is always unique near the relatively stable state of system. Theoretical analysis is qualitatively consistent with the experimental results obtained.

A detached eddy simulation method with the turbulent separation is presented to simulate and analyze the characteristics for the flow around a circular cylinder with a turbulent boundary layer separation and its lift/drag coefficients in a weakly conductive fluid at a high subcritical Reynolds number 1.4×105 under an electromagnetic force. The results show that the electromagnetic force can increase the fluid kinetic energy near the turbulent boundary layer, delay the turbulent boundary layer separation of the flow around the circular cylinder, and weaken the intensity of the large-scale eddy in the turbulent flow around the circular cylinder in the streamwise and spanwise directions, reduce the time-average drag, and inhibit the lift fluctuation amplitude on the circular cylinder. Moreover, after the electromagnetic force parameter reaches a certain critical value, the turbulent boundary layer separation disappears and the jet phenomenon appears in the wake of the circular cylinder, so that the electromagnetic force produces the thrust action on the circular cylinder and the negative drag occurs, and the lift fluctuation amplitude declines to almost zero and the lift phenomenon on the circular cylinder disappears.

A laboratory wave-making method is developed for the internal solitary wave under the condition of giving its amplitude produced by oppositely and horizontally pushing two vertical plates placed separately in the upper- and lower-layer fluids of a large-scale density stratified tank where based on the Miyata-Choi-Camassa (MCC) theoretical model, the layer-mean velocities of the upper- and lower-layer fluids induced by the internal solitary wave are used as the velocities of the two plates. On this basis, a series of experiments is conducted to explore the applicability conditions for internal solitary wave theories with stationary solutions which are Korteweg-de Vries (KdV), extended KdV (eKdV), MCC and modified KdV (mKdV) models in a two-layer fluid of finite depth respectively. It is shown that for the nonlinear parameter ε and the dispersion parameter μ defined by the total water depth, there exists a critical dispersion parameter μ0, in the case of μ μ0, the KdV model is applicable for ε ≤μ, the eKdV model is applicable for μ ε ≤√μ, as well as the MCC model is applicable for ε > √μ. However, in the case of μ ≥ μ0, the MCC model is still applicable for a wide range of ε. Furthermore, for the case where the ratio of depth between the upper- and lower-layer fluids is not close to its critical value, the mKdV model is mainly applicable for the case where the amplitude of the internal solitary wave is close to its theoretical limiting amplitude, however, the MCC model is also applicable for such a case. The investigation quantitatively characterizes the applicability conditions for four classes of internal solitary wave theories, and provides an important theoretical foundation for what kinds of theories can be chosen to model internal solitary waves in the ocean.

A numerical investigation on the evolution mechanism and characteristics of the submerged laminar round jet in a viscous homogenous fluid is conducted by using the computational fluid dynamics method based on the incompressible Navier-Stokes equation. Three non-dimensional parameters for the mushroom-like vortex structure, including the length of the jet L*, the radius of the mushroom-like vortex R* and the length of vortex circulation d*, are introduced and the variation characteristics of these parameters with the non-dimensional time t* are quantitatively analyzed. Results show that there exist three distinct stages in the formation and evolution procedures of the mushroom-like vortex structure, including the starting, developing and decaying stages. In the starting stage, L* and d* increase linearly with t*, while R* approximately remains to be a constant; in the developing stage, a considerable self-similarity is confirmed, and L*, R*, d* display the same proportional relationship to t*1/2 regardless of the variations of Reynolds number and injection duration; in the decaying stage, L* and R*are approximately proportional to t*1/5, while d* nearly levels off as a constant. Moreover, velocity characteristics at the secondary backflow point and the momentum and geometry centers, the distribution features of the vertical vorticity, as well as the vorticity-stream function relationship are analyzed for the mushroom-like vortex structure.

Based on the consistence principle for the severe slugging formation condition, a computational fluid dynamics (CFD) method is proposed for numerically simulating the gas-liquid severe slugging in a pipeline-riser system by converting the three-dimensional pipeline-riser system into a two-dimensional equivalent one. Numerical simulation is conducted for the gas-liquid flow patterns of the severe slugging in a declination pipeline-riser system according to the experimental cases presented in the reference, and variations of characteristics of the flow parameters with gas-liquid superficial velocity due to such a severe slugging are obtained, including period, pressure fluctuation and blowout time, the numerical results are in good agreement with the experimental results. Moreover, the theoretical methods are obtained of determining the gas superficial velocity at the riser inlet, the gas volume fraction in the riser and the average velocity at the riser outlet during the severe slugging blowout stage, and the variations of characteristics of these flow parameters with time are presented. Besides, a theoretical predicting method of determining the flow patterns in the riser during the severe slugging blowout stage is further proposed, and the theoretical results are consistent with the CFD numerical results. The CFD method could save much time and computing resource, and the theoretical methods could be used to predict the damage of severe slugging quickly.

Experiments are conducted for the characteristics of both body and wake-generated internal waves due to a revolution body (aspect ratio 7:1) in a stratified fluid with a pycnocline where the revolution body is horizontally towed in forward and backward ways. Results show that the body-generated internal waves are a type of stationary multiple-mode Lee wave structures, and the wake-generated internal waves are a type of non-stationary quasi-Lee structures produced by the large-scale coherent structure in the turbulent wake acting as a moving excitation with respect to the towed revolution body. The transition between Lee and quasi-Lee waves occurs at a critical Froude number Frc which is shown to be linearly dependent on the aspect ratios of the revolution bodies where the wave patterns for FrFrc are dominated byLee waves,whereas the wave patterns forFrFrcare dominated by quasi-Lee waves,Fr is the Froude number for the characteristic diameter of the revolution body. For the quasi-Lee waves, the dimensionless peak-to-peak amplitudes linearly increase with Fr and the Froude numbers relative to the correlation velocities remain at a relatively constant value of approximately 0.8 regardless of the aspect ratios. Moreover, the head and the tail shapes of the revolution body have no remarkable influence on the critical Froude number Frc, as well as both the Froude numbers relative to the correlation velocities and the dimensionless peak-to-peak amplitudes of the quasi-Lee waves.

Experiments were carried out in a stratified fluid with a momentum source modeled by a horizontally moving jet. The generation mechanism was investigated and the evolution characteristics were analyzed for the quasi-two-dimensional dipolar vortex streets generated by such a moving momentum source in the stratified fluid. The conditions of different Reynold and Froude number (Re, Fr) combinations under which the dipolar vortex street can be generated by moving momentum source were determined by use of data based on the present series of experiments. Moreover, the dependence of the dimensionless formation time and inverse dimensionless average wavelength of the dipolar vortex street on Fr for different values of Re was obtained. The results show that these parameters are independent of Re and approximately follow some exponential laws of Fr.

An equivalent mass source is modeled as excited sources of internal waves generated by the displacement of fluid and the wake of vortex shedding or turbulence due to the body moving in a stratified fluid, a mathematical model for the vertical displacement of internal waves generated by such an equivalent mass source is presented by use of both the associated eigenvalue problem and the Fourier transform method, and the method for estimating the speed, length and diameter of the equivalent mass source is proposed. Calculations based on the proposed method are performed for the generation of internal waves by a towed sphere in a stratified fluid, and the numerical results are in good agreement with the available experimental results of Robey in the kinematic wave patterns and the maximum peak-to-peak amplitude of the internal waves. Such a method may have potentially important applications.

A simple fast algorithm is proposed for visualizing multiple three-dimensional (3D) obstacles from the knowledge of the near-field measurements of the scattered field for point source field.The proposed algorithm is achieved by using the 3D contour plot of the solution of a linear system as a visualization of the scattering object.An attractive feature of this algorithm is that it avoids the use of the iteration technique and does not need any priori knowledge about the geometry and physical properties of scatterers.In addition,the algorithm only requires the knowledge of the near-field measurements of the scattered field at a finite number of incident and observation points distributed over some limited area.Some numerical examples based on synthetic near-field data are given to show the practicality and efficiency of the inversion approach.