東華大學圖書館 |

Interaction Between Dense Gravity Currents and Internal Waves /

Record Type:	Electronic resources : Monograph/item
Title/Author:	Interaction Between Dense Gravity Currents and Internal Waves // Yukinobu Tanimoto.
Author:	Tanimoto, Yukinobu,
Description:	1 electronic resource (230 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 83-09, Section: B.
Contained By:	Dissertations Abstracts International83-09B.
Subject:	Lasers. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=29003867
ISBN:	9798209787563

Interaction Between Dense Gravity Currents and Internal Waves /
Tanimoto, Yukinobu,

Interaction Between Dense Gravity Currents and Internal Waves /Yukinobu Tanimoto. - 1 electronic resource (230 pages)

Source: Dissertations Abstracts International, Volume: 83-09, Section: B.

Gravity currents are flows driven by horizontal density gradients. A dense gravity current is a specific type of gravity current where the density of the fluid is higher than that of the ambient, making the gravity current negatively buoyant. When the receiving ambient is stratified, the ultimate fate and transport of a dense gravity current becomes more complex. We consider a two-layer stratification, representative of a stratified water column in lakes and in the near-coastal environment.As the gravity current propagates, mixing through entrainment and detrainment can generate fluid of intermediate densities between that of the gravity current and the receiving ambient. Stratification in the ambient provides multiple levels of neutral buoyancy, allowing the intermediate density fluid from the dense gravity current to detach from the main flow. Furthermore, a stratified receiving ambient can support internal waves, which propagate along density interfaces. Internal waves can be generated by a dense gravity current, where momentum is transferred from the gravity current to the interface. This thesis focuses on the interaction between a dense gravity current and internal waves: first, on the generation of internal waves by a gravity current in a two-layer stratification at short and long time scales; and second on the interaction and role of oncoming internal waves to modify the propagation of a dense gravity current.The first portion of this thesis focuses on a series of laboratory experiments to investigate the dynamics of a finite-volume, dense gravity current propagating down an incline into a quiescent twolayer stratification. The dynamics of the gravity current propagating in the upper layer is similar to the homogeneous ambient case, which has been extensively studied by others in the past. Upon reaching the pycnocline, the gravity current splits into two parts: one that penetrates the pycnocline as an underflow, and one that inserts itself into the ambient stratification at the pycnocline as an interflow. The splitting of the gravity current is compared to theoretical hypothetical outcomes, and is observed to be dependent on the initial gravity current Richardson number (Rip), formulated as the ratio of excess density and the ambient stratification. For low Rip, the underflow is more dominant, and for higher Rip, the interflow is more dominant. However, with increasing Rip the splitting behaviour eventually becomes independent of Rip Additionally, two dierent waves are identified based on the dominant flow after the splitting of the gravity current. An underflowdominated regime causes the pycnocline above the head of the gravity current to elevate, leading to the internal wave crest locked in with the propagation of the head of the gravity current. In an interflow-generated regime, the gravity current generates an internal wave that is launched ahead of the interflow and moves faster than the speed of either part of the gravity current.The internal waves generated by the gravity current continually interact with the geometry of the basin at longer time scales. As either of the internal waves reaches the basin wall and is reflected, the baroclinic response of the ambient stratification was found to be analogous to a stratified system forced by a constant wind stress. The baroclinic response of the system to the forcing is characterized by a Wedderburn-like number, which was found to be linearly and inversely proportional to the initial Rip. The motions generated were characterized as having a low frequency internal surge, and higher frequency internal waves that are generated following the surge. As the higher frequency internal waves approach the topographic slope, they steepen and break above the surge. The overall mixing eciency from the breaking events was calculated and found to be low when compared to similar previous studies.The second portion of this thesis focuses on the interaction between a dense gravity current and oncoming internal waves. The facility is modified to allow for a constant flow gravity current, such that the e↵ects of multiple internal waves on a steady state gravity current can be investigated. The propagation of the gravity current in the presence of oncoming internal waves are compared to the same case without oncoming internal waves. Using simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), spatially and temporally resolved velocity and density fields show that the interflow is generated by a combination of detrainment of the gravity current and the pycnocline to guide the intermediate density fluid into interflow. The mass flux of the interflow is observed to be reduced by oncoming internal waves, as waves reduce the velocity of the interflow. Phase averaging the velocity and density profiles showed that the reduction in velocity is not uniform across the wave period.

English

ISBN: 9798209787563Subjects--Topical Terms:

535503
Lasers.

Interaction Between Dense Gravity Currents and Internal Waves /
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502 $b Ph.D. $c Stanford University $d 2021.
520 $a Gravity currents are flows driven by horizontal density gradients. A dense gravity current is a specific type of gravity current where the density of the fluid is higher than that of the ambient, making the gravity current negatively buoyant. When the receiving ambient is stratified, the ultimate fate and transport of a dense gravity current becomes more complex. We consider a two-layer stratification, representative of a stratified water column in lakes and in the near-coastal environment.As the gravity current propagates, mixing through entrainment and detrainment can generate fluid of intermediate densities between that of the gravity current and the receiving ambient. Stratification in the ambient provides multiple levels of neutral buoyancy, allowing the intermediate density fluid from the dense gravity current to detach from the main flow. Furthermore, a stratified receiving ambient can support internal waves, which propagate along density interfaces. Internal waves can be generated by a dense gravity current, where momentum is transferred from the gravity current to the interface. This thesis focuses on the interaction between a dense gravity current and internal waves: first, on the generation of internal waves by a gravity current in a two-layer stratification at short and long time scales; and second on the interaction and role of oncoming internal waves to modify the propagation of a dense gravity current.The first portion of this thesis focuses on a series of laboratory experiments to investigate the dynamics of a finite-volume, dense gravity current propagating down an incline into a quiescent twolayer stratification. The dynamics of the gravity current propagating in the upper layer is similar to the homogeneous ambient case, which has been extensively studied by others in the past. Upon reaching the pycnocline, the gravity current splits into two parts: one that penetrates the pycnocline as an underflow, and one that inserts itself into the ambient stratification at the pycnocline as an interflow. The splitting of the gravity current is compared to theoretical hypothetical outcomes, and is observed to be dependent on the initial gravity current Richardson number (Rip), formulated as the ratio of excess density and the ambient stratification. For low Rip, the underflow is more dominant, and for higher Rip, the interflow is more dominant. However, with increasing Rip the splitting behaviour eventually becomes independent of Rip Additionally, two dierent waves are identified based on the dominant flow after the splitting of the gravity current. An underflowdominated regime causes the pycnocline above the head of the gravity current to elevate, leading to the internal wave crest locked in with the propagation of the head of the gravity current. In an interflow-generated regime, the gravity current generates an internal wave that is launched ahead of the interflow and moves faster than the speed of either part of the gravity current.The internal waves generated by the gravity current continually interact with the geometry of the basin at longer time scales. As either of the internal waves reaches the basin wall and is reflected, the baroclinic response of the ambient stratification was found to be analogous to a stratified system forced by a constant wind stress. The baroclinic response of the system to the forcing is characterized by a Wedderburn-like number, which was found to be linearly and inversely proportional to the initial Rip. The motions generated were characterized as having a low frequency internal surge, and higher frequency internal waves that are generated following the surge. As the higher frequency internal waves approach the topographic slope, they steepen and break above the surge. The overall mixing eciency from the breaking events was calculated and found to be low when compared to similar previous studies.The second portion of this thesis focuses on the interaction between a dense gravity current and oncoming internal waves. The facility is modified to allow for a constant flow gravity current, such that the e↵ects of multiple internal waves on a steady state gravity current can be investigated. The propagation of the gravity current in the presence of oncoming internal waves are compared to the same case without oncoming internal waves. Using simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), spatially and temporally resolved velocity and density fields show that the interflow is generated by a combination of detrainment of the gravity current and the pycnocline to guide the intermediate density fluid into interflow. The mass flux of the interflow is observed to be reduced by oncoming internal waves, as waves reduce the velocity of the interflow. Phase averaging the velocity and density profiles showed that the reduction in velocity is not uniform across the wave period.
546 $a English
590 $a School code: 0212
650 4 $a Lasers. $3 535503
650 4 $a Fluid mechanics. $3 528155
650 4 $a Mechanics. $3 525881
650 4 $a Optics. $3 517925
650 4 $a Physics. $3 516296
690 $a 0204
690 $a 0346
690 $a 0752
690 $a 0605
710 2 $a Stanford University. $e degree granting institution. $3 3765820
720 1 $a Koseff, Jeffrey Russell $e degree supervisor.
720 1 $a Fringer, Oliver B. $e degree supervisor.
720 1 $a Ouellette, Nicholas $e degree supervisor.
773 0 $t Dissertations Abstracts International $g 83-09B.
790 $a 0212
791 $a Ph.D.
792 $a 2021
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