Turbulence and mixing are generated by the shear between two counter-flowing layers in hydraulically controlled buoyancy-driven exchange flows through a constriction. From direct measurements of the density distribution and the amount of turbulent mixing in steady laboratory exchange flows we determine the overall efficiency of the mixing. For sufficiently large Reynolds numbers the mixing efficiency is 0.11(±0.01), independent of the aspect ratio and other details of constriction geometry, in good agreement with a scaling analysis. We conclude that the mixing in shear flows of this type has an overall efficiency significantly less than the maximum value widely proposed for stratified turbulence.

[1] Buoyancy-driven, hydraulically controlled two-layer exchange flows were established in a laboratory flume with a lateral constriction and a bottom sill placed at its midpoint. The shear flow was unstable within the constriction, and the overall amount of turbulent mixing was determined from measurements of vertical density profiles after the exchange. The potential energy changes are used to evaluate the mixing efficiency (or flux Richardson number), defined as the fraction of the available potential energy released into kinetic energy that is converted to potential energy through irreversible mixing. Mixing efficiencies asymptote at large Reynolds numbers to values of 8% for a sill with sloping sides and 11% ± 1% for both a weir with vertical sides and the same constriction with no bottom topography. The latter is identical to the value previously reported for shorter, smoothly varying lateral constrictions. Thus, all controlled exchanges examined here undergo less mixing than implied by the commonly assumed maximum turbulent mixing efficiency of 20%, and the actual efficiency is insensitive to the form of the topography. This result implies that calculations of the global energy balance of the ocean may need to be reevaluated.

<p class="16">Covid-19 spurs teachers to carry out online learning. This study aimed to analyze the improvement of students' critical thinking skills through online learning based on Android-based PhET Simulation. This research was Quasi-Experimental using one group pre-test and post-test design involving 27 students of class VIII who program science subjects at MTs Alif Laam Miim Surabaya, Indonesia. The research instrument used critical thinking skills tests that have met valid and reliable requirements. Before and after learning activities, students were given a pre-test and a post-test. The data collected was analyzed descriptively quantitatively. The results showed that there was an increase in students' critical thinking skills with an average N-gain of 0.57, in the medium category. The highest N-gain was achieved in the "interpretation" indicator, which is 0.83 in the high category and the lowest in the "analysis" indicator, which is 0.39 in the moderate category. This study concluded that learning science by using an Android-based PhET Simulation can improve the critical thinking skills of junior high school students.</p>

The linear shallow-water approximation is commonly used to describe tsunami propagation, where the wave is assumed as a long surface gravity wave. The evolution of wave height during its propagation from offshore to onshore is a classic problem. When arriving at a shoreline, the increased wave height causes severe destruction on infrastructures and fatalities. This problem has then been an important issue within the context of disaster risk reduction as it gives rise to the importance of tsunami run-up prediction. Using maximum run-up data from past events, we tested the applicability of the Green's law based on shoaling only to calculate run-ups and found that the basic Green's law was in doubt. Then, we examined energy density conservation involving refraction effect but no dissipation and derived a simple formula for parameterizing run-up height. Detailed descriptions on factors affecting run-ups, such as complex bathymetry and topography are not yet considered in the current study. The aim of this study is therefore to determine whether the modified Green's law is applicable for tsunami run-up prediction using local water depths as external parameters and ray spacing widths in the normal direction of wave fronts related to refraction. The results are consistent with the measured run-ups, where approximately 70% of total points of observations confirm the modified Green's law with a reasonable accuracy.

Peran analogi sebagai salah satu strategi pengajaran sains dalam menunjang proses belajar mengajar di sekolah sangat besar. Strategi ini dapat digunakan sebagai suatu metode alternatif untuk memecahkan kebuntuan komunikasi belajar antara guru dan siswa, khususnya bila siswa menghadapi kesulitan belajar dalam hal memahami materi ajar baru namun memiliki kemiripan alur berpikir dengan materi ajar sebelumnya. Sebagai basis berpikir metode pengajaran dengan analogi, artikel ini akan memperkenalkan dan mendiskusikan dua konsep utama, yaitu: konsep rujukan, materi ajar yang sudah dipahami sebelumnya oleh siswa, dan konsep target, materi ajar yang akan dipahamkan kepada siswa. Pembahasan kedua konsep tersebut akan disertai dengan beberapa contoh kasus dalam pengajaran fisika di sekolah. Tidak tertutup kemungkinan bagi para guru di kelas untuk memanfaatkan metode alternatif ini sebagai salah satu cara untuk meningkatkan keterampilan berpikir siswa. Dengan demikian, pengayaan materi ajar tidak saja dapat dilakukan melalui latihan soal berulang dan berjenjang, melainkan juga dapat dengan memperkenalkan paradigma baru agar diperoleh spektrum pemahaman materi ajar yang lebih luas dan komprehensif.